The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Disclosures relate to compositions and methods for treating subterranean formations, in particular, oilfield stimulation compositions and methods using ionically modified polymer crosslinked with a borate source to viscosify the treatment fluid.
High volumes of formation fracturing and other well treatment fluids are commonly thickened with polymers such as guar gum, the viscosity of which is greatly enhanced by crosslinking with boron and/or a metal such as chromium aluminum, hafnium, antimony, etc., more commonly a Group 4 metal such as zirconium or titanium. In reference to Periodic Table “Groups,” the new IUPAC numbering scheme for the Periodic Table Groups is used herein as found in Hawley's Condensed Chemical Dictionary, p. 888 (11th ed. 1987). Sometimes guar is modified with ionic groups to facilitate hydration of the polymer and to improve crosslinking with metal complexes. Ionic modification of the polymers can reduce the time it takes to dissolve the dry polymer at the well site, and improve both the ultimate gel strength and the thermal persistence of the gel upon crosslinking with a metal crosslinking complex.
It is well known that metal-crosslinked polymer fluids can be shear-sensitive after they are crosslinked. In particular, exposure to high shear typically occurs within the tubulars during pumping from the surface to reservoir depth, and can cause an undesired loss of fluid viscosity and resulting problems such as screenout. As used herein, the term “high shear” refers to a shear rate of 500/second or more. The high-shear viscosity loss in metal-crosslinked polymer fluids that can occur during transit down the wellbore to the formation is generally irreversible and cannot be recovered. We use the term “persistent gels” herein to refer to such irreversibly crosslinked aqueous polymers.
High shear sensitivity of the metal crosslinked fluids can sometimes be addressed by delaying the crosslinking of the fluid so that it is retarded during the high-shear conditions and onset does not occur until the fluid has exited the tubulars. Because the treatment fluid is initially cooler than the formation and is usually heated to the formation temperature only after exiting the tubulars, some delaying agents work by increasing the temperature at which gelation takes place. Bicarbonate and lactate are examples of delaying agents that are known to increase the gelling temperatures of the metal crosslinked polymer fluids. Although these common delaying agents make fluids less sensitive to high shear treatments, they may at the same time result in a decrease in the ultimate fluid viscosity. Also, the common delaying agents may not adequately increase the gelation temperature for the desired delay, especially where the surface fluid mixing temperature is relatively high or the fluid is heated too rapidly during injection.
In contrast, a boron-crosslinked polymer solution is substantially tolerant to high shear rates, such as those experienced in the wellbore tubulars, without damaging the performance of the gel. Although there is a loss of viscosity in borate-crosslinked systems during high shear, that viscosity is regained rapidly after the substantial reduction or cessation of shearing and the borate crosslinks are re-formed.
In some treatment systems, borate crosslinkers have been used in conjunction with metal crosslinkers, e.g. U.S. Pat. No. 4,780,223. In theory, the borate crosslinker can gel the polymer fluid at a low temperature through a reversible crosslinking mechanism that can be broken by exposure to high shear, but can repair or heal after the high shear condition is removed. The shear-healing borate crosslinker can thus be used to thicken the fluid during high shear such as injection through the wellbore while the irreversible metal crosslinking is delayed until the high shear condition is passed, i.e. usually after entry into the formation or fracture. A high pH, e.g. 9 to 12 or more, is usually used to effect borate crosslinking and in some instances as a means to control the borate crosslinking. For example, the pH and/or the borate concentration may be adjusted on the fly in response to pressure friction readings during the injection so that the borate crosslinking occurs near the exit from the tubulars in the wellbore. The metal crosslinker must of course be suitable for use at these pH conditions and must not excessively interfere with the borate crosslinking.
For polymers that are ionically modified for improved hydration and ultimate gel properties, crosslinking can sometimes be difficult, especially with crosslinkers such as borates that do not form strong bonds to the polymer crosslinking sites. Some anionically and/or cationically modified polymers tend to expand or uncoil in aqueous media due to the repulsion of like charged moieties on the polymer backbone, reducing overlapping to the extent borate crosslinking does not occur. The rheology profile of carboxymethylhydroxypropyl guar (CMHPG) in the presence of borate crosslinker is shown in FIG. 1. However, the presence of 2 wt % KCl in the otherwise identical CMHPG-borate solution can effectively screen the anionic charges with electric bi-layers to decrease the charge intensity, and in turn decrease the repulsions between charged polymer chains. Charge screening in this manner can collapse the polymer chains and achieve overlapping for borate crosslinking to occur as also shown in the rheology profile in FIG. 1.
Standard 2 wt % KCl brines are also sometimes conveniently used as a clay stabilizer when drilling or treating certain formations wherein clay is prone to swell from aqueous exposure. Unfortunately, while the use of KCl or other high ionic strength brines can obtain both clay stabilization and effective borate crosslinking of CMHPG, the final gel strength and thermal persistence of the metal-crosslinked gels can be adversely affected by the high ionic strength.
On the other hand, tetramethyl ammonium chloride (TMAC) is also a common clay stabilizer, particularly when it is desired to use a low ionic strength treatment fluid without impacting ultimate gel strength and/or thermal persistence at higher formation temperatures; however, the use of TMAC in place of KCl is ineffective to charge screen CMHPG in that borate crosslinking does not occur when TMAC is used in a low conductivity fluid medium. As used herein, a low conductivity medium is one having a conductivity which measures less than 10 mS/cm, preferably less than 5 mS/cm and especially about 2 mS/cm or less, or having a KCl concentration less than 0.5 wt % by weight of the liquid phase.
It has been proposed to hydrophobically modify ionic polymers using an oppositely charged surfactant having a relatively long hydrophobic group, where the surfactant forms an ion-pair association with the polymer resulting in a hydrophobically modified polymer having a plurality of hydrophobic groups, as described in published application US 20040209780. The hydrophobic groups in adjacent hydrophobically modified polymers are said to form micellar associations by the further addition of surfactant, thereby forming crosslinks through the micellar associations and increasing the viscosity of the fluid system. However, applicant's investigation, as reflected in FIG. 4 below, suggests that longer hydrophobic groups and/or higher surfactant concentrations are needed for successful micellar association crosslinking.
What is needed in the art is a system and well treatment method in which an anionically or cationically charged polymer with a clay stabilizer can be crosslinked in a low-conductivity fluid medium, by borate and/or metal crosslinkers, without significantly adversely impacting final gel strength and/or thermal persistence of the crosslinked gels.